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Sir Isaac Newton first presented his three laws of motion
in the "Principia Mathematica Philosophiae Naturalis" in 1686. His second law
defines a force to be equal to the change in momentum
with a change in time.
Momentum is defined to be the mass m of an object times its
velocity
V.
Let us assume that we have an airplane at a point "0" defined by its
location X0 and time t0. The airplane has
a mass m0 and travels at velocity V0.
The airplane is subjected to an external force F and moves to a point "1",
which is described by a new location X1 and time t1. The mass and
velocity of the airplane change during the flight to values m1 and V1.
Newton's second law can help us determine the new values of V1 and m1,
if we know how big the force F is.
Let us just take the difference between the conditions at point "1" and the conditions
at point "0".
F = (m1 * V1  m0 * V0) / (t1  t0)
Newton's second law talks about changes in momentum (m * V) so, at this point, we can't
separate out how much the mass changed and how much the velocity changed.
We only know how much
product (m * V) changed.
Let us assume that the mass stays a constant value equal to m.
This assumption is pretty good for an airplane, the only change in mass would be for the
fuel burned between point "1" and point "0". The weight of the fuel is probably
small relative to the weight of the rest of the airplane, especially if we only
look at small changes in time.. If we were discussing the
flight of a
baseball,
then certainly the mass remains a constant. But if we were discussing the flight of a
bottle rocket,
then the mass does not remain a constant and we can only look at changes in momentum.
For a constant mass m, Newton's second law looks like:
F = m * (V1  V0) / (t1  t0)
The change in velocity divided by the change in time is the definition of the acceleration a.
The second law then reduces to the more familiar product
of a mass and an acceleration:
F = m * a
Remember that this relation is only good for objects that have a constant mass.
This equation tells us that an object subjected to an external force will accelerate and that the
amount of the acceleration is proportional to the size of the force. The amount of acceleration
is also inversely proportional to the mass of the object; for equal forces, a heavier object will
experience less acceleration than a lighter object. Considering the momentum equation, a force
causes a change in velocity; and likewise, a change in velocity generates
a force. The equation works both ways.
The velocity, force, acceleration, and
momentum have both a magnitude and a direction associated with them.
Scientists and mathematicians call this a
vector quantity.
The equations shown here are actually vector equations and
can be applied in each of the
component directions. We have only looked at one direction,
and, in general, an object moves in all three directions (updown, leftright,
forwardback).
The motion of an aircraft resulting from
aerodynamic forces, aircraft
weight, and thrust
can be computed by using the second law of motion.
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Newton's Laws of Motion:

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